Octopuses can taste what their arms touch.

Exactly right. Mollusks like Cephalopods (squid, octopuses, cuttlefish), Gastropods (snails, slugs) and Bivalves (oysters, clams) are far more adaptable than people realize. There are plenty of gastropods who have evolved lungs and they can survive in the semi-arid climate of Los Angeles and Ventura Counties without being desiccated. 

So, you may ask, why are Vertebrates, not Mollusks, the dominant life forms on land and sea? There are two main reasons for that seeming anomaly. First of all, vertebrates have light weight internal skeletons which support the organs and to which powerful muscles can be attached. Secondly, while both mollusks and vertebrates have circulatory systems, vertebrate blood is iron-based hemoglobin while mollusk blood is copper-based hemocyanin. Hemoglobin delivers more energy than does hemocyanin. This all means that vertebrates are more agile and powerful than mollusks. Mollusks just aren't that strong and this fact accounts for the supremacy of vertebrates over mollusks. Don't believe the bullshit of giant octopuses drowning ships or carrying off half-clothed nubile females. They just aren't that strong. They're quite smart but also quite weak. 

However, there are other questions that might be asked. For instance, what would happen if Covid-19 should mutate into forms that would eliminate all humans, all Vertebrates, all Deuterostomes (Chordates and Echinoderms)? Would the hyper-intelligent octopuses conquer the world? Into what forms would they develop given 200 million years to evolve? In answer, I say that Octopuses would rule this planet and devolve into some of the forms pictured below. 

They might vaguely be reminiscent of extinct humans with a military and a bureaucracy:



They might evolve into a myriad of forms filling the niches abandoned by birds, mammals, reptiles, fish and other extinct genera:



What do you think? Are the creatures portrayed here realistic? If Octopuses should develop civilization, will they create some kind of religion? How would it differ from what we believe about things that are numinous? Note, I've answered your questions, so you should do the same and respond to mine. Try to show a spark of decency, this one time.